A known method of embedding auxiliary data is disclosed in U.S. Pat. No. 5,748,783. In this prior art method, an N-bit code is embedded through the addition of a low amplitude watermark which has the look of pure noise. Each bit of the code is associated with an individual watermark which has a dimension and extent equal to the original signal (e.g. both are a 512×512 digital image). A code bit “1” is represented by adding the respective watermark to the signal. A code bit “0” is represented by refraining from adding the respective watermark to the signal or, alternatively, by subtracting it from the signal. The N-bit code is thus represented by the sum of up to N different watermark (noise) patterns.
When an image (or part of an image) in, say an issue of a magazine, is suspected of being an illegal copy of an original image, the original image is subtracted from the suspect image and the N individual watermark patterns are cross-correlated with the difference image. Depending on the amount of correlation between the difference image and each individual watermark pattern, the respective bit is assigned either a “0” or a “1” and the N-bit code is retrieved.
A drawback of the prior method is that N different watermark patterns are to be added at the encoding end, and N watermark patterns are to be individually detected at the decoding end.